I posted my issue on corvetteforum.com but since its a C4 TPI corvette I think there is alot of knowledge at TGO that could be applicable.

I recently finalized my switch from a Superram intake to an Edelbrock Victor E EFI and there is now an issue that I cannot resolve. The engine is a fully built 383 L98 with a Vortech V2 s-trim blower and a standalone EFI system

I have been driving the car on a few occasions (tuning) but after about 20 minutes, the engine leans out to AFR 18-20 (from 12-14) and it start to run really crappy and the exhaust gets super hot. Yestereday it even started to come smoke out of the shifter boot (prob the glass fiber heatshield)!

Until this happens, the car is running fine with normal AFR.

This is not a WOT thing but happens at idle and cruising. I have checked fuel pressure with and without the vacuum hose attached and the pressure is rock steady at around 40psi. No excessive coolant nor intake air temperature either.

After shutting down the engine for about 20 minutes, the symptons goes away until the car heats up again.

Measured temperature on fuel hoses, rails and regulator and it varies between 120 and 140F. Does not look very extreme to me.

The fuel cap is always slightly open since I have deleted the coal canister hence no pressure in the tank.

Im running an in-tank walbro 340 and and inline smaller walbro (dont recall the exact name but the setup was designed and supplied by Greg at Blowerworks).

Was a year or maybe two since I replaced the fuel filter but since the car is running perfect around 20 minutes I cannot see the filter to be the issue.

The engine is in"open loop" so the lambda sensor only measure the oxygen and has no influence on the air/fuel ratio.

Could it be bad injectors?

 

almost sounds like vapor lock

 

Is this likely to happen (I mean the fuel is under pressure)?

Can it be related to bad ignition?

 

vapor lock is when your fuel turns from liquid to vapor when it gets to hot has nothing to do with ignition your leaning out sounds more like a fuel problem then ignition

 

Is it so,you have no pressure at fuel tank->lower fuel boiling point->fuel vaporized at tank when goes hot?

 

What aftermarket EFI are you running and what distributor? GM or cheapy aftermarket?

 

The EFI system is an Autronic SMC :http://carterrace.com/front/index.ph...&Itemid=45#smc

I have the stock GM distributor though its modified to produce a cylinder 1 sync signal (oem module is no more).

I put in a new coil and distributor cap but no change-still goes lean.

 

It sounds like a odd problem, are you sure there is no vacuum leaks when warm? Also, is one of the pumps or pump relay failing when warm? One pump may give you a good psi reading but if one has failed, the other pump pushing or pulling through the other one will cut down on the fuel volume.

The reason I ask about the distributor and EFI is, I once delt with a customers 57 chevy that had a aftermarket EFI and some cheapy aftermarket distributor parts.
It would run fine then all of a sudden, it was like the programing was erased and it would run really messed up, until it cooled off.
Turns out, the EFI was set up to handle the GM distributor parts and not the aftermarkets. The outputs between the aftermarket Dizzy and EFI just weren't compatible at all.

 

Well, I have not really looked into potential vacuum leaks but I have got some engine starter fluid. Next time I will use it to establish if there are any leaks.

I am running a MSD 6a box and it looks like this could also be a potential problem, especially when exposed to heat. Mine is under the coolant expansion reservoir and it get quite hot. I have borrowed a brand new msd box I will try.

I will look into the fuel pumps and relays after I have ruled out any ignition problems and vacuum leaks.

 

Ok, I have now used starter fluid to identify vacuum leaks. Tried both cold engine as well as hot (when it has started to misfire).
Result: nothing

Now its time to change the MSD box.

 

Try running it without the MSD. Just had a friends car puke a 5th MSD box. Car would run but it sucked.

 

Unfortunately I had to remove the OEM ignition module from the HEI distributor when I hooked up the Autronic so I cannot override the MSD.however, I have a brand new MSD 6al I will give a try.

 

Tried the new msd box but another failure. The misfire is still there when I start it up the engine when hot and keeping the rpm at idle and slightly above. There is possibly a slight improvement with the new box and the misfire tends to decrease / almost goes away when I give it some throttle and get the car going and increase rpms.

 

..... What is your fuel pressure ? ... Before and during the problem zone ... and what coolant temps are you seeing when it starts to lose Air fuel ratio ? It sounds to me like a possible tuning issue where the ECM is taking away some other coolant relative fueling as the engine warms up .........

 

Fuelpressure steady at around 40psi-no change when the car starts to misfire.
Coolant temperature around 190F when things start to get bad. However on another logged run, the same temp did not induce the misfire.

There is basically one table that influence mixture based upon input from coolant and intake air temp. I have tried some different settings on this but did not help.
My original setting should not be that far away since i only changed intake.

 

I have now tried various extreme settings on intake air and coolant temp compensation-no change.

Disconnected brake booster without any change (suspected internal vacuum leak)-no change.

Maybe leaving the car on jackstands for 6 months during winter caused some contaminents to be released and it now clogs the injectors?

 

..... Don't get carried away with your diagnostics ... keep it simple and start from the beginning ... what year car ? what size and mfr injectors ? have you Ohm checked the injectors hot ? a vacuum leak would affect cold operation as well ... have you checked the Engine Coolant Temp sensor ? Is the engine properly grounded to the chassis/battery ? Heat will also affect HEI distributors ... coil , ICM (it needs to have the correct heat sinking grease under it) and in very rare cases the pick-up coil ....... look very closely at all the electrical connections that may have been disturbed by the intake manifold change , perhaps a pin has slipped out of place ....... Have you talked to Greg @ Blowerworks ? ......

 

Its à 1990 Corvette but its only the oilpan and engine block is remaining from the Old L98. The injectors are Siemens 57lbs low impedance. I have tested a couple when hot and they checked out ok (2,9 ohm) but I can controll the remaining ones.
I can log basically all engine parameters and coolant temp is one. Did not show any strange or excessive values when going from Normal running to misfire.Since the values seen by the ECU is normal I dont think the sensor is the problem.

The HEI is gutted and the starwheel, pickup and ignition module is removed and I have already tried a new ignition coil, distributor cap and MSD box.

It only happens when engine is hot, shouldn't bad grounding and loose pins give the same effect when engine is cold?

 

Sounds like something electrical is getting hot and shutting down.

 

Agree. Fuel pumps? Fuel pump relays? Injectors? Im considering removing the injectors and send them for ultrasonic cleaning after I have checked out the relays.

Even though I don't think its the cause, I might change fuel filter just to check the inside of the old one. I really hate it since I get sprayed with fuel everytime I do it.

 

..... If the fuel pressure is constant I would rule out the supply end of the fuel circuit ... delivery (injectors) can show performance drops when hot , but that should be evident in the injector Ohm check ... if ONE injector is bad , it can draw down the rest at least on batch fire systems ... cleaning the injectors won't hurt anything except the time it takes to get it done ... I think you need to revisit all the changes that were made in doing the intake swap and make sure you didn't overlook something ... heat can affect resistance values in ground connections ... on an EFI engine you can't have too many ground wires .....

 

The Autronic I´m using is a sequential fired system. Prob not very high possibility to several injectors gone bad simultanously, I think I´ll wait doing anything with the injectors.

I have messed around mostly with fuel delivery when I did the intake swap and viewing things from that stand point, this is likely the issue.

However, as a next step I will change spark plugs just to rule out everything ignition related.

 

Interested to see the results.

This happens even at idle as well once hot?

I am having a similar issue, but my FP starts jumping around so I know I am experienceing vapor lock.

I will check back often.

Good luck and don't give up!

 

Can you see if the BPW is changing?

 

Nufnuff: are you running EFI and experience vapor lock? I' ve seen quite contradictory opinions if this is even possible when running the fuel pressure required for EFI and yes, it happens at idle as well.
Gta324: what is BPW?

 

"Base pulse width" opening time of the injector

 

Maybe Im missing something but to my understanding since im running in open loop mode, the ECU just goes on whats in the map and the associated pulse width.

 

correct, and I'm not familiar with Autronic but looking at the BPW when the AFR gets skyhigh you could see if its somthing in the tuning thats affecting the BPW calculation. If the BPW is the same there could be a problem with "delivery" the fuel.

(some fuel adders/subtractor in the tuning can affect the AFR alot when you running big injectors and have short BPW specially at idle)

Also check the timing with a strobe when the AFR gets high so it looks OK.

( I've had TPI cars home with missfire when hot and you could see the timing starts to jump around when hot-> bad ign module)

 

I looked in my logged runs, when normal and when misfiring and BPW is approximately the same (about 2ms/1%dutycycle).

Havent thought about checking real timing, good idea. Sofar only been looking at commanded values. I use two hall sensors (GT101 and http://www.bbautomacao.com/home_hall...r_cyhme56.html), maybe one of those gone bad? I will get my strobe light and check next time.

 

I have now changed all sparkplugs, put in a new fuel filter and added alot of extra heat protection on the fuel lines where they are closest to the headers-wish me luck...

 

When my MAP sensor failed I went very lean on WB. Car barely could run at 17/1

 

Sounds like a mechanical issue rather than in the tune. You stated that this all started when switched from a SR to a single plane, correct, nothing else what touched? If your idling in Open Loop, and the air/fuel is steady when cool, but then goes lean as air temps and coolant get hot, then its somewhere in your fuel delivery because as air gets hot, it is less dense, and you would show rich and not lean if your fuel delivery is good...

Did you burp the coolant system when swapping intakes? Do you have the fuel pressure regulator connected to the proper port? Are you sure your base timing and spark angle reference match after pulling the distributor after the swap? Did you check your fuel pressure when keyed off to see if pressure drops immediately...?

 

Only the intake is changed and this is when the problems started.

I haven't burped the system but fluid level looks ok.

The vacuumhose that goes from the regulator is hooked up to a connection at the intake elbow base.

The fuel pressure only drops a couple of psi when key is off. I have hotwired the fuelpump so I can run it without the engine running. The idea was to be able to circulate the fuel to cool it off-no change despite running the fuel pump about a minute. Still misfire.

Checked the spark angle today with a timing light. Quite unstable but every now and then it shows the correct value (tested at idle). I understund this is due to the multipel spark from the MSD Box.

I have checked position of the rotor and its spot on towards the no1 tower on the distributor cap.

I have now also changed spark plugs, put in a new fuel filter and added a lot of extra heatprotection of the fuel lines. The result...? Another failure

The misfire seem to occur even faster today despite the above.

Maybe changing rotor and checking distributor gear (Im running a HV oil pump)?

 

......... Where is the Manifold Air Temp Sensor located ? ..........

 

On the intake elbow after the TB but before the intake manifold. I have tested several settings (including some extreme ones) on the ait compensation (fuel enrichment/leaning of fuel) without any improvement. I have logged IAT and nothing extreme during misfire.

 

Just throwing it out there. Maybe the intake manifold gaskets. As things get hot and expand you might get a leak under the manifold that cant be found with started fluid. Since it started with the intake swap, it should have to do with the intake or new throttle body.

 

What side do you have the WB? Is it possible to change side?

Do you have a IR thermo gun?

( try to figure out if is one bank that missfire or both or just a few cylinders )

Since you have stable Fuel pressure and the BPW doesnt change it's sounds like it getting to much air suddenly......

 

The WB is on the driver side collector. I can hook up another WB on the other side I (will borrow one).
I have tried the IR gun briefly but it was very difficult to get readings from the passenger side headers (totally cramped). Can make another try.

I´m still a little stuck on the fuel side of things. After some googling, it looks like vapor lock indeed can happen in EFI equipped cars, even though its pretty uncommon.

Also, it might be the case that released gas inside the plumbing will not give an abnormal fuel pressure reading and you might get tricked by the appearantly "perfect" fuel pressure on the display (its an Autometer electronic).

As mentioned above, I will do the easy tasks of checking the dist gear and change rotor before I start digging into the more work intense debugging things (checking for leaks under manifold, stuck valves, fuel pump etc).

I don't know if it's relevant but my superram intake base was slightly milled on the side since the block was zero decked. Now I have put on a non-milled intake, there´s no strange sound that indicate dist gear not set but I´m now starting to grasp for anything...

 

(for the price of a WBO2 cost today I think dual is a cheap insurance if something happens on a supercharged engine)

yes vapor lock can happen

I know one car here in sweden who has (probably) vapor lock.... But on that car the fuel pressure starts to jump when it gets very hot....

have you tried just giving it more fuel ( change ve-table) when it happens?

 

I have added extra fuel at the specific cell in the fuel table when Im idling. It looked like the AFR got somewhat less lean but the misfire is still there.

 

I have had a look at the old spark plugs and they basically all look the same (white on the electrode and black on the base). Can take a pic if you like. Looks like something is affecting all 8 cylinders.

 

Try a good known MAP see what happens. Hada MAF go bad yrs ago scanned fine but when hot would go dead lean

Warm misfire....re ohm those injectors. Said you checked 2 and they ohmed at 2 and 9? not good would def. cause a misfire and lean condition

highly doubt vapor lock in an efi application

Gotta be one of the 2 or an ign piece getting hot im no expert though

 

The MAP sensor is inside the computer box that's inside the glove compartment. I have logged several runs when misfiring and the MAP values look fine. Everything works fine for the first 10-20 minutes so I think the sensor works (its not getting hot inside the glove comp).

I recall I measured resistance on 6 injectors when hot and all 8 when cold. All measured around 2.5ohms when cold which is according to spec for low impedance injectors (Siemens 3102 57lbs/h).

Im currently checking valve lash but sofar it looks ok. I will also rewire all injectors (this was planned anyway). Will also reroute fuel lines to the rails to run them in series (now parallell).

 

Electrical parts get hot from current and normal use, not just placement.

 

..... Is this system sequential fuel injection ? If so , any chance you have the injector wiring mixed up or out of the firing order ? ... Does idle speed change much when it goes lean ? ......

 

Yes its sequential. Unfortunately I have now removed all connectors and wiring from the injectors so no way to find out.

I recall idle speed was approximately the same when misfiring.

 

Found the cause of the misfire, it was two injector wires/clips gone bad so two injector shot down when everything got hot. Thank you for all great ideas and support.